This invention relates to the design of an ophthalmic lens series and, especially, to a molded polycarbonate ophthalmic lens series.
The design of a series of ophthalmic lenses providing optimal correction with a limited number of front and back lens curvatures has been a subject of extensive study. Articles discussing the problems of designing a lens series or individual lenses thereof include J. K. Davis, "Stock Lenses and Custom Design," Am. J. Optom. & Arch. Am. Acad. Optom. 44(11):776-801(1967); J. K. Davis, H. G. Fernald & A. W. Rayner, "The Design of a General Purpose Single Vision Lens Series," Am. J. Optom. & Arch. Am. Acad. Optom. 42(4):203-36(1965); and J. K. Davis, H. G. Fernald & A. W. Rayner, "An Analysis of Ophthalmic Lens Design," Am. J. Optom. & Arch. Am. Acad. Optom. 41(7):400-21(1964). Recent patents disclosing such lens series include U.S. Pat. Nos. 3,960,442 and 3,169,247, issued to J. K. Davis and H. G. Fernald, and U.S. Pat. No. 3,434,781, issued to J. K. Davis, H. G. Fernald and A. W. Rayner.
Traditionally, the individual lenses of a series are designed by selecting the front and back curvatures so as to optimize one or more error criteria at selected off-axis points while providing no intended refraction error at the optical center along either major meridian. Such error criteria as have been formulated to date, however, have not taken full account of the intended wearer's viewing habits for close and distant vision and, for this reason, are not entirely satisfactory. Further, the specification of zero error at the optical center often introduces an unnecessary constraint, particularly in correcting off-axis power errors.
Moreover, these previous disclosures have generally emphasized the optimization of the design of the individual lenses making up the series and have not given full consideration to optimizing the relationship between the curvatures of the respective lenses. This relationship of individual curvatures is important, however, for several reasons. First, obvious considerations of economy dictate that a lens series be generated from families of front and back curves that are as small as possible consistent with satisfactory off-axis correction.
Second, since image magnification is primarily controlled by the front curvatures, simply optimizing this curvature for each individual lens is likely to result in appreciable magnification imbalances between pairs of left and right lenses of slightly different power. In the prior art it has been common practice to optimize a front curve for a selected prescription and then use that particular front curve over a range of prescriptions, resulting in steps of one to two diopters between adjacent front curves of a series. This practice introduces magnification differences between the lenses of the two eyes when there is a slight difference in prescription that falls on a point in the series where front curves are changed. Further, this stepping system introduces compromises in the design at these borderlines between front curve selections.
Finally, the relationship between the lens curvatures is important even from the point of view of on-axis performance. Traditionally, lens grinding tools and molds for plastic lenses are ground to "standard ophthalmic curves". These curves have dioptric values in even steps of 1.00, 0.25 and sometimes 0.125 D. The problem arises in that no currently used ophthalmic optical material has an index of refraction of 1.53, for which the tools have been designed; ordinary spectacle lens glass has an index of 1.523; high-index glass has an index of 1.70. The common hard resin plastic used to make lenses has an index of approximately 1.50, and some of the newer materials have higher indices. For example, polycarbonate material has an index of approximately 1.586.
In practice, manufacturers of semi-finished blanks take into account tool index in designing the front curve of the semi-finished blank, so that the curve on the blank which is nominally +10.25 D is not exactly +10.25 D, but a curve which takes into account the fact that the concave curve has a slightly different value than the label on the tool. There is a further compensation on the front surface due to the fact that front convex surfaces gain power when referred through the thickness to the ocular surface. Thus, there are two compensations that enter into the radius of the front surface, one due to the use of standard 1.53 tools for grinding the concave surface and the other due to the thickness effect.
As a result of using the 1.53 tool index system, if one wishes to change prescription by 1.00 D by using the same given front curve while maintaining the thickness, one cannot easily do it since there are no tool steps of 1.00 D true power. At 1.00 D, this is not serious. If one wishes to change by more than 1.00 D, the errors begin to be significant. When these inherent errors are added to those normally attributed to manufacturing tolerances or inaccuracies of production methods, there is often waste in laboratories. Also, prescription errors result which may be within standard tolerances but still result in lenses which are not as accurate as they may otherwise be. This inaccuracy is particularly true in the case of astigmatic corrections where the cylinder is calibrated in the standard 1.53 curves. The result is that almost no cylinder prescription which is finished with the standard tools can have the specified value.